The invention relates to a vacuum plasma generator for feeding a plasma discharge for the purpose of treatment of workpieces in a vacuum chamber according to the claims, as well as a method for the production of a layer by reactive deposition out of a plasma.
Electrical feeding devices for operating gas discharges or plasma discharges for vacuum processes have already become variously known. Such electrical feeding devices are also known as generators in the technical world. It is important here to be able to exercise good control over the operating conditions, since the nature of the plasma discharge and the plasma process tied thereto make special demands. Apart from the voltage input, the current and the discharge power, the behavior during flash-overs in the plasma, also denoted as arcs, as well as possible shortcircuits must be especially well controlled. Such arc discharges, or arcs, occur repeatedly during the operation of plasma discharges and require special measures. In particular when coating with the aid of a sputtering process, also denoted as sputtering, such flash-overs occur and can negatively affect the quality of the product or even make coating impossible. Such problems occur especially severely if poorly conducting layers or even dielectric layers are to be generated. To deposit such layers, for example by means of magnetron sputtering, the so-called reactive sputtering is preferably employed. Herein, for example, the target of a magnetron source is supplied with negative DC voltage to attain a so-called DC plasma, which is maintained for example with a carrier gas such as argon, and the ionized argon particles sputter the target with the aid of an impact process. The sputter target is here developed metallically, thus is a conductive material, and during the sputtering is brought to reaction with additive reactive gas, for example oxygen, whereby subsequently an oxidic layer can be deposited. Due to the covering of the surrounding surfaces, but also of the target itself, with the oxidic layer, arc problems are herein encountered. The nonconducting coated surfaces become charged during the DC sputtering process and the undesirable arc discharges are generated.
Until now attempts at alleviating the problem have included, for example, detecting such arcs and subsequently briefly switching off the generator in order to run up the plasma again. Another known solution comprises carrying out high-frequency discharges in the MHz range or medium frequency discharges in the kHz range or superimpositions of such AC voltages with respect to a DC basic discharge. A further known possibility includes feeding the plasma during reactive sputtering processes with the aid of a DC pulsed generator. The AC voltage component contained in the pulse feeding, also referred to as AC component, and/or the pulse interspaces make possible the periodic discharge of the undesired charging in the vacuum space. Since the dielectric, respectively nonconducting, layers in these reactive processes are relatively thin, for example in the micrometer range or less, relatively low frequencies in the kilohertz range are sufficient for this purpose. Such DC-pulsed configurations are denoted as unipolar pulse configurations. In order to further advance the discharge process on these dielectric layers, it is helpful if the electrode during the interspace time is not only switched free of the feeding but rather is shortcircuited relative to the anode or ground. Hereby the discharge process is purposefully positively affected. Further acceleration of the discharge process can be attained if in the interspace time, which means when no argon ions are accelerated with the negative pulse toward the target, even to switch the voltage briefly into the positive range in order to generate a greater discharge gradient for electrons and to accelerate the discharge process once more. Consequently bipolar pulses are already employed here, however these preferably not being symmetric and the negative pulse time area being greater than the positive one in order to ensure high sputter rates. Typical discharge voltages in magnetron sputter configurations are within the range of a few hundred volts, typically approximately in the range from negative 400 to 800 volts. Depending on the layout of the magnetron source and the operating pressures, the voltage ranges can also be wider or narrower. For the accelerated discharge of dielectric chargings it is here sufficient to move briefly in the interspace time to a few volts or some ten volts positive.
Known furthermore is also the possibility of operating two electrodes as cathodes, where these are alternately switched cathodically and the other time anodically, meaning they are operated with a bipolar pulse generator. This configuration has also been referred to as Twin Mag. Herein, each electrode is alternately once the cathode and subsequently the anode. This is an especially advantageous configuration for reactive sputtering processes. Compared to the known high frequency-controlled AC plasma configurations, the unipolarly or bipolarly operated DC-clocked DC configurations have the advantage of high rates. The complicated reactive process, in addition, can be better controlled. Modern generators of this type are realized as controllable DC generators, where in particular the switched mode power supply technique, also known as DC voltage converter or known in English as switched mode power supply or converter configurations, have become known. As is customary in electronics, switched-mode power supplies of this type are structured such that first the AC voltage mains, for example the three-phase mains with 3×400 V AC 50 Hz is rectified in conventional manner to generate a DC voltage. The generated DC voltage down from the mains in this case is approximately 550 V and is subsequently converted with the aid of a switched-mode power supply or a so-called DC-DC converter to the desired voltage values. For this purpose the DC voltage is essentially chopped under control with the aid of semiconductor switches and via a transformer correspondingly step-transformed up or stepped-down, and the configuration can conventionally be controlled, for example in terms of pulse width, via the semiconductor switches in order to be able to generate a variably settable output voltage. It is understood that the configuration can also be regulated to the correspondingly desired output voltages, currents or powers.
Such switched-mode power supplies or DC-DC converters have become known as various types and by now there are many variation forms of implementation possibilities. For operating a plasma generator with unipolar starting pulses for the plasma feeding, such DC-DC converters are employed directly by controlling or clocking these at the primary side via switching transistors and at the output side a corresponding pulsing DC voltage is generated. In the case of higher powers and for the generation of bipolar output pulses, the use of semiconductor bridge circuits, as with transistors or thyristors, has become pervasive. In this case the bridge is utilized in known manner for the purpose of inverting the polarity of the DC voltage fed into the bridge. The load or the plasma discharge path is here always switched directly into the semiconductor bridge branch. Expenditures for the realization of these known configurations is considerable. Especially with respect to protective measures of the semiconductors at the high operating voltages and especially also at high powers, special protective measures are required for the semiconductors. A further problem consists therein that in the event of flash-overs or shortcircuits fast transient processes take place, which lead to induced voltage superelevations and to high current peaks, which can additionally destroy the sensitive power semiconductors. Operation with reactive processes, in particular with poorly conducting materials or even insulation layers at high powers or at high pulse powers, and especially with defined pulse rise times, has until now only yielded limited success with the known pulse generators.